1,721,028 research outputs found
Nudged elastic band calculations of the (4H)XSi hydrogarnet type defect in Mg2SiO4forsterite
No full textFirst-principles calculations based on density functional theory (DFT) using the generalized gradient approximation (GGA) were performed to assess the energetic barriers separating different topological configurations of the (4H)XSi hydrogarnet type defect in Mg2SiO4 forsterite with the climbing image nudged elastic band (CI-NEB) method. Barrier heights are low (<0.6 eV) with respect to typical ac- tivation energies observed for H-diffusion but more comparable to those for electrical conductivity of H2O-bearing nominally anhydrous minerals. As can be expected, hydrogen bonding to O atoms both within the defect and belonging to adjacent tetrahedra plays a fundamental role in the stability of each configuration. Saddle points along the minimum energy path (MEP) typically correspond to the transition of one hydrogen bond breaking to form a new hydrogen bond such that one or more OH bonds have shifted in direction without themselves breaking. MEPs show that slightly out-of-plane torsional hopping from one configuration to another can reduce the height of the barrier. We illus- trate several different reaction coordinates between symmetry equivalent pairs of configurations and non-symmetry related pairs that can result in an effective means of local charge transport by shifting the center of mass of the (4H)4+ cluster within the defect site without proton transfer to an interstitial site. Especially at low temperatures in the absence of thermally activated processes that result in the breaking of stronger chemical bonds, these types of configurational transformation mechanisms are likely to be important contributors to the dielectric behavior of nominally anhydrous silicate minerals and also affect both electrical conductivity and electrical conductivity anisotropy when investigated by AC methods such as impedance spectroscopy. The NEB method can also be used to examine more effective charge and mass transport processes that involve the dissociation of the hydrogarnet defect into more complex chemical species, which might involve similar hydrogen bond breaking and forming processes observed in this study along with more significant atomic displacements
Electrical conductivities of pyrope-almandine garnets up to 19 Gpa and 1700°C
No full textElectrical conductivities of polycrystalline garnets ranging in chemical composition from almandine
(Fe3Al2Si3O12) to pyrope (Mg3Al2Si3O12) were measured at 10 GPa and 19 GPa at temperatures ranging from 300 to 1700 °C using complex impedance spectroscopy in a multianvil device. Mössbauer
spectroscopy of each sample was carried out both before and after the electrical measurements to characterize the oxidation state of Fe in the almandine bearing garnets. Similar to the behavior of other ferromagnesian silicates, the substitution of Fe for Mg along this compositional join dramatically increases electrical conductivity, but this compositional effect is reduced with increasing temperature. Conductivities increase with increasing total Fe content, as the average Fe2+-Fe3+ distance decreases. At 10 GPa, activation energies for conductivity vary smoothly with composition and increase rapidly toward the pyrope end-member composition, where it reaches a value of 2.5 eV. The results are consistent
with an electrical conductivity mechanism involving small polaron mobility in the Fe-bearing
garnets at 10 GPa. At 19 GPa, however, there is virtually no change in the activation energy as a function
of Fe-Mg substitution for the pyrope-rich garnets. These higher pressure measurements reflect a mechanism involving oxygen related point defects, as conductivities increase with pressure at constant
T for each garnet, and the effect of pressure is greater for the more Mg-rich garnets. The data also allow for a more quantitative evaluation of the effect of chemical composition, specifically Fe-Mg substitution, on the electrical conductivity profi le of the mantle, using a recently developed laboratory-
derived model. We apply the model using these data to a portion of the transition zone between 520 and 660 km, in which we vary the garnet composition from Py100 to Py85Alm15. Although only
a minor effect on bulk mantle conductivity results, we conclude that the overall garnet composition may, however, be important in characterizing the magnitude of any EC discontinuity with respect to the above-lying mantle
Electrical conductivity of hydrous single-crystal San Carlos olivine
No full textAbstract n° MR41A-089
What drives large-scale basic Plinian eruptions? Insights from textural and rheological studies on the Pozzolane Nere eruption (Colli Albani, Italy)
No full textDiagenetic thermal evolution of organic matter by Raman spectroscopy
No full textVitrinite reflectance is considered one of the most reliable indicators of thermal maturity in sedimentary basins and is widely used to calibrate numerical models that assess hydrocarbon generation/expulsion from source rocks. Nevertheless, ambiguities in vitrinite reflectance can occur from variations in composition and preservation state of organic facies, temperature/pressure conditions and the abundance of organic matter resulting in incorrect or uncertain assessments of petroleum systems. This study presents an original application of Raman spectroscopy to assess the diagenesis - catagenesis of kerogen between the immature and mid-mature stages of hydrocarbon generation. Kerogens were isolated from 33 intervals of drill cuttings from a 5 km thick Oligocene-Miocene siliciclastic section of the Malembo Formation (Lower Congo Basin, Angola). Their Raman spectra were obtained and derived parameters were compared to depth and to previously reported equivalent vitrinite reflectance values. The best correlations between thermal maturity and Raman parameters were found for D-G band distance; FWHM of the G band; D/G area ratio; RA2 ratio (calculated as S + Dl + D/Dr + Gl + G band ratio) and the D/G width ratio. The Raman parameters were not influenced by varying kerogen composition (mixed type II/III). The technique offers the potential to reduce the risk of source rock thermal maturity assessment even when organic facies rich in amorphous organic matter are present. © 2017 Elsevier Lt
Electrical conductivity of hydrous wadsleite
No full textIn situ complex impedance spectroscopy of H2O-bearing wadsleyite was performed in a multianvil apparatus at 14 GPa at temperatures up to 950 °C in order to determine electrical conductivity. With increasing H2O content in wadsleyite the electrical
conductivity increases at a rate higher than observed in previous studies. The activation enthalpy in the temperature range studied
where proton conduction dominates is low (0.66 eV) suggesting an inevitable crossover to small polaron conduction at moderately
higher temperatures, depending on H2O concentration. Although the solubility of H2O in wadsleyite is significant for a silicate mineral (> 3 wt%), the presence of more than trace dissolved H2O in wadsleyite is likely to result in a conductivity too high compared to recent estimates of transition-zone conductivity. The use of complex impedance spectroscopy shows that the frequency dependence
of electrical properties is very different in the case of H2O-bearing silicate phases. At frequencies below 1000 Hz complex impedance spectra contain strong features which likely result from the sample–electrode interface such that including the low-frequency data would lead to artificially low conductivities
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